The present invention generally relates to a power supply for a magnetron suited to a microwave oven, in which its current source unit is miniaturized to reduce the weight, and an output control unit is also diminished in size for simplification in order to decrease the costs, and more particularly, to a magnetron power supply wherein a life span of a cathode (filament) of the magnetron is not shortened even when the output of magnetron is varied in a wide range including an intermittent stoppage.
In a method of stepping up a voltage of a commercial AC frequency with a transformer and rectifying it for the purpose of obtaining high DC voltages to be impressed on an anode of the magnetron which generates a high frequency for a microwave heater, the transformer tends to be bulky and heavy. In recent years, there has been utilized a high frequency heater using a so-called inverter system power supply which is arranged in such a way that a commercial alternating voltage is rectified to obtain a direct voltage, and the direct voltage is inverted into a higher frequency voltage than the commercial AC frequency by opening and closing a switching element preparatory to be stepped up by a transformer which can be small both in size and in weight, and a high DC voltage obtained by rectifying this alternating voltage once again is impressed on the anode of the magnetron. In accordance with the inverter system power supply, it is possible to readily control the output with a high efficiency by controlling an ON/OFF time ratio (duty ratio) of the switching element.
A prior art high frequency heater using the inverter system power supply is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 79345/1977. This type of high frequency heater incorporates an anode current detector for detecting an anode current of the magnetron and a duty control unit for controlling an opening/closing duty ratio of the switching element in accordance with a signal transmitted from the anode current detector.
In the case of the high frequency heater for use with a microwave oven or the like, in the great majority of cases, an AC high voltage output across a secondary winding of the transformer are employed as an anode voltage source of the magnetron by converting the output into a direct voltage by means of a half-wave or full-wave voltage doubler rectifier circuit composed of a combination of diodes and a capacitor. Where a magnetron anode current value of such a circuit is detected on the basis of the above-described conventional technique, a current value sharply fluctuates in time despite of being the direct current. Hence there is typically employed such a current transformer that it is relatively easy to establish the insulation between the high voltage circuit to be measured and the current value detecting circuit, and a loss in electric power is small.
The magnetron anode current flowing on the primary-side of the current transformer has already been, however, rectified and become a unidirectional current. Therefore, a core section has to be sufficiently large so as not to saturate the core forming a magnetic circuit of the current transformer. This arrangement is especially necessary when employing the full-wave rectifier circuit.
On the part of the current value detecting circuit, since no DC component is present in the output of the current transformer, a zero-level adjustment has to be performed by means of a DC restoration circuit, which leads to an increment in the number of components.
FIG. 1 is a circuit diagram schematically illustrating one example of the conventional high frequency heater in which the high frequency given by the inverter system power supply serve as the inputs of the transformer, and the half-wave voltage doubler rectifier circuit is employed on the secondary-side of the transformer. The reference numeral 1 designates a commercial AC power supply; 10 represents a rectifier circuit; 20 denotes a transformer; 30 stands for a switching element; 31 is a driving circuit; 40 a capacitor; 50 a diode; 60 a current transformer; 70 a voltage comparator for comparing an output voltage of a current value detecting circuit 90 for detecting an electric current to a magnetron with a reference voltage value which is preset corresponding to a desired output; 80 a magnetron; 81 a heater current source for the magnetron; R1 to R5 resistances; D1 a diode; Q1 and Q2 transistors; and C1 represents a capacitor. If the current transformer is used as detecting means, though a measurement object is the rectifier magnetron anode current (direct current), no DC component is present in the output, i.e., the electric current of the current value detecting circuit. For this reason, after effecting the zero-level adjustment by the DC restoration circuit, the anode current has to be detected by clipping it with the diode D1. As a result, the circuit requires a good number of components including Q2, R4, C1 and so on.
The current flowing on the primary-side of the current transformer is a pulsating, but unidirectional direct current. Hence, the section of the magnetic circuit of the current transformer has to be large so that no magnetic flux is saturated and magnetic flux interlinking with a secondary winding varies sufficiently. For this reason, the core of the current transformer becomes large for the output, and it follows that the transformer is heavy and large in size. This brings about an increase in cost.
The output voltage of the current value detecting circuit is compared with the reference voltage value which is preset corresponding to the desired magnetron anode current value by means of the voltage comparator 70. If there flows an electric current whose current value is higher than the desired value, the driving circuit 31 is controlled by the output of the voltage comparator 70 to decrease a duty ratio of the switching element. If it is smaller than the desired value, however, the driving circuit 31 is controlled to increase the duty ratio. Videlicet, the driving circuit 31 and the voltage comparator 70 are combined to form a control circuit for the switching element 30.
In case of using the conventional magnetron power supply for obtaining a high anode voltage of the magnetron by use of a commercial frequency transformer, the output of the magnetron has heretofore been adjusted by intermittently disconnecting the voltage source. However, if a cathode filament of the magnetron is turned on and off, especially when decreasing the cycle of intermittent operations, a life span of the filament is outstandingly diminished. In general, a transformer for the filament is separately provided to constantly heat the filament. The commercial frequency transformer is heavy and large. To cope with this, as disclosed in, e.g., Japanese Utility Model Laid-Open Publication No. 107396/1987, there has recently been developed a light-weight inverter system magnetron power supply constructed in such a manner that the direct voltage obtained by rectifying a commercial frequency voltage is converted into a higher frequency voltage by using the switching element; the transformer is diminished both in weight and in configuration by employing an inverter circuit for inverting the direct voltage into an alternating voltage having a high frequency than the commercial frequency; and simultaneously the microwave output of the magnetron is adjusted by varying an opening/closing time ratio of the switching element.
The above-described prior art aims at applying the magnetron to the high frequency heating in a microwave oven or the like. This purpose is accomplished either by controlling the microwave output given to the materials to be heated on the basis of the intermittent operations at intervals of several ten seconds by use of the commercial frequency voltage source or by controlling the microwave output changing the opening/closing time ratio of the switching element by use of the inverter system power supply.
Excepting the microwave oven, a microwave generating device which employs the magnetron includes industrial devices such as a plasma etching device, a stroboscope type high luminance light emitting device, a fine spot heating device in semiconductor processing. Some of these devices require high-speed switching of the output in addition to precisely controlling the microwave output value.
A power supply designed for switching only the high anode voltage of the magnetron has already been put into a practical use, wherein the filament transformer is separately provided. This type of power supply is, however, large in size and expensive.